This invention relates to systems for the generation of voltages such as deflection voltage generators in displays configured to be controlled by digital signals and, more particularly, to the monitoring of the performance of the voltage generation.
An exemplary voltage generator is found in one form of a display utilizing a cathode ray tube (CRT) for displaying symbols and patterns at various locations on the face of the display. The voltage generator includes a character generator and X-Y position voltage circuitry. The symbols and patterns are conveniently generated by means of a character generator wherein a selected symbol is produced in response to a digital control signal. The position of the displayed symbol is set by a pair of digital-to-analog converters which convert digital signals representing the X and the Y coordinates to analog voltages which are applied to the CRT deflection circuitry. Signals produced by the character generator are combined with the coordinate signals at the input to the deflection circuitry so that the symbols are thereby positioned on the display. In many applications, such displays are utilized to present output data from a computer in which case the computer provides the digital control signals for designating the specific symbols and their locations on the display.
In the foregoing display system, it is seen that a set of voltages is generated in response to a digital control signal. Similarly, in automated milling machinery and other industrial processors, a preprogrammed set of analog voltages is produced in response to a digital control signal.
Analog currents may be produced instead of the analog voltages. For example, deflection circuitry frequently utilizes analog signals, such as analog currents, to drive a pair of deflection coils for directing the electron beam of the CRT.
A problem arises in that such deflection circuitry and automated process control circuitry is subject to drift depending on the length of time that the circuitry is in operation. To correct for the drifting, in display deflection circuitry, attempts have been made to monitor the signals applied by the deflection circuits to the deflection coils, in the case of magnetic deflection, or to the deflection plates in the case of electrostatic deflection. However, such attempts have utilized equipment which is unduly complicated in that it requires the use of much additional circuitry not normally found in displays.